Bioko Island, called Fernando Po until the 1970s, is the largest island in the Gulf of Guinea - 2,017 square kilometers (779 sq mi). It is shaped like a boot, with two large volcanic formations separated by a valley that bisects the island at its narrowest point. The 195-kilometer (121 mi) coastline is steep and rugged in the south but lower and more accessible in the north, with excellent harbors at Malabo and Luba, and several scenic beaches between those towns.

On the continent, Rio Muni covers 26,003 square kilometers (10,040 sq mi). The coastal plain gives way to a succession of valleys separated by low hills and spurs of the Crystal Mountains. The Rio Benito (Mbini) which divides Rio Muni in half, is unnavigable except for a 20-kilometer stretch at its estuary. Temperatures and humidity in Rio Muni are generally lower than on Bioko Island.

Annobon Island, named for its discovery on New Year's Day 1472, is a small volcanic island covering 18 square kilometers (6.9 sq mi). The coastline is abrupt except in the north; the principal volcanic cone contains a small lake. Most of the estimated 1,900 inhabitants are fisherman specializing in traditional, smallscale tuna fishing and whaling. The climate is tropical—heavy rainfall, high humidity, and frequent seasonal changes with violent windstorms.

The climate of both the continental region and the islands is typically equatorial, with high temperatures, heavy rainfall, and much cloud cover most of the year. Local variations are due to differences in altitude and proximity to the sea. The wet seasons in the continental region are from February to June and from September to December. Rainfall is higher on the coast than inland. In Bata the rainiest months are September, October, and November, with rainfall averaging more than 94 in (2,388 mm) a year. At Calatrava, farther south on the coast, it sometimes reaches 180 in (4,572 mm). Inland, however, rainfall diminishes; Mikomeseng, for example, receives only about 58 in (1,473 mm). The average annual temperature is about 79 °F (26.1 °C) and is fairly constant throughout the year. The temperature maxima are somewhat lower than in Bioko. The relative humidity, however, is higher than in Bioko. Bioko has a rather debilitating climate. The so-called dry season lasts from November to March, and the rest of the year is rainy. The average annual temperature of about 77 °F (25 °C) varies little throughout the year. Afternoon temperatures reach the high 80s °F (low 30s °C) and drop to only about 70 °F (21.1 °C) at night. Most of the time the sky is cloudy and overcast. Extreme rainfall occurs in the south, with rain brought by monsoon winds amounting to about 450 in (11,430 mm) a year around San Antonio de Ureca.